1. Field of the Invention
The present invention relates to a telecommunications network system, and more particularly to a passive optical communication network system using an optical code division multiplexing scheme.
2. Description of the Background Art
In recent years, a demand for telecommunications has been rapidly increased as the Internet diffuses, for example. High-speed large-capacity telecommunications networks have accordingly been built, using optical fibers. Additionally, in order to develop large-capacity telecommunications, an optical multiplexing system is considered important which transfers a set of optical pulse signals for plural channels over a single optical fiber transmission line.
As one of the optical multiplexing systems, the optical code division multiplexing (OCDM) has been developed. The OCDM has such advantages that physical resources such as time slots and wavelengths are not occupied exclusively by one channel, that passive devices can be used as encoder and decoder components, and that synchronization may not be established between channels. Therefore, by using the OCDM, physical resources are not occupied by one channel to thereby save telecommunications resources, and passive devices can be used to directly encode and decode optical signals to thereby accomplish high-speed processing. Additionally, since a function for synchronizing channels with each other is not required, systems may be configured simply. Due to these advantages, the OCDM is expected to be a solution applicable to fiber-to-the-user systems for use in broadband networks.
In fiber-to-the-user optical networks, one optical line terminal (OLT) on the premises of a communication station is connected to one or more optical network units (ONUs) on the premises of subscriber or subscribers, so that subscribers transmit and receive information to and from an external network through optical transmission between the ONUs and the OLT. The transmission lines are configured by optical fibers, which are connected to a star coupler as an inexpensive passive device to thereby establish multiple access between the plural ONUs and the OLT. Specifically, such a network system comprises an OLT interconnected by an optical fiber to a star coupler, which is interconnected to ONUs by a corresponding plurality of optical fibers. Optical networks thus configured only by passive devices such as optical fibers and star coupler are called passive optical networks (PONs), some of which have so far been standardized as a demand for telecommunications and innovation in technology develop. Particularly, schemes based on time compression multiplexing, such as Broadband-PON and Ethernet-PON, have been put into practice.
Japanese Patent Laid-Open Publication No. 2009-24733 and N. Kataoka, et al., “Field Trial of Duplex, 10 Gbps×8-User DPSK-OCDMA System Using a Single 16×16 Multi-Port Encoder/Decoder and 16-Level Phase-Shifted SSFBG Encoder/Decoders”, Journal of Lightwave Technology Vol. 27, No. 3, Feb. 1, 2009, pp. 299-305, disclose examples of a PON system using the OCDM. In the examples, the OLT includes a single optical encoder called a multi-port optical encoder for use in encoding or decoding signals on all channels, in order to simplify the configuration of the system. The ONU includes a super-structured fiber Bragg grating (SSFBG) and a circulator for use in encoding or decoding only signals on a necessary channel, in order to simplify the configuration of the system. The network has a star-shaped topology. On the downstream traffic, i.e. traffic from the OLT to the ONUs, the power of an OCDM signal transmitted from the OLT is split by the sole star coupler to arrive at all the ONUs. On the up-going traffic, i.e. traffic from the ONUs to the OLT, the power of single-channel encoded signals transmitted from all the ONUs are combined by the sole star coupler to arrive at the OLT in the form of single OCDM signal.
It is preferable to adapt a communication station having its capacity sufficient for accommodating an increased number of subscribers since the OLTs are suppressed from increasing in number and hence the space for installation and power consumption are decreased accordingly. Therefore, one of the requirements for designing an optical network may be involved in increasing the loss budget of the system. In order to increase the loss budget, a loss caused by an extended transmission distance and an increased number of branches has to be suppressed by (A) raising the transmission power, (B) lowering the optical sensitivity, and (C) raising the availability of power of an optical signal. The measure (A) causes the power consumption of a transmitter to increase, and the measure (B) causes devices configuring a receiver to be so sophisticated as to be expensive. Thus, the measure (C) will be described below.
On the downstream traffic of the above-described OCDM-PON, the multiplexed signal transmitted from the OLT for simultaneous delivery on plural channels is split by the star coupler. Then, when the split optical signals arrive at the ONUs, the multiplexed signals are decoded in order to separate the channels. Therefore, if all the channels included in the multiplexed signal have their power equal to one another, the optical signal has its power decreased (branching loss) due to the star coupler to a value equal to a fraction of the number of branches from transmitted to received, and further due to a decoder to a value equal to a fraction of the number of multiplexed channels. The number of branches in a star coupler is usually fixed when installed first. Therefore, once the number of branches is fixed when a system is installed first, the branching loss remains as it is even if subscribers or multiplexed channels increase or decrease in number.